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We have a radio comms site with multiple equipment. Originally the site was set up with a positive ground at the rectifiers. We have some new equipment that is powered from the rectifier shelf, that will not work well with a positive ground. We were thinking of changing one of the rectifiers to a negative ground. My question is would this cause a problem?

In my mind, having a positive ground and negative ground connected to the same main site ground just seems wrong, but is it? Would there really be a conflict here or not?

All voltage is DC, with exception of input power to rectifier shelves (120AC).

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In my mind, having a positive ground and negative ground connected to the same main site ground just seems wrong, but is it? Would there really be a conflict here or not?

If you are using bridge rectifiers then you'll have a big problem with a set of rectifiers for positive ground and a set of rectifiers for negative ground: -

enter image description here

Shown in red is the almost-short-circuit current that will flow for positive half-cycles of the AC waveform (shown as a circle). It's the same for negative half-cycles too but through different diodes.

What springs to my mind is the use of a DC-to-DC convertor that isolates out a DC supply that is equal to your current DC supply voltage. Because it is an isolated voltage (internal switching power supply and transformer) either of its two leads can be conected to ground.

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Having different equipment that uses different grounding is OK as long as there isn't a direct DC connection between the different equipment. Put another way, if each piece of equipment is isolated from the others with only the 120 VAC power feed in common, then it doesn't matter.

It also doesn't matter if all the external signals are referenced to the common ground. The fact that one box runs from a negative supply internally and the other from a positive supply internally is just that: internal. It would be surprising to me if external signals were not either ground referenced or floating, because doing otherwise at more than a few volts would be a safety hazard.

A block diagram with how exactly power is hooked up would help.

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  • \$\begingroup\$ Thanks. I dont have a diagram at this time, but the rectifier shelves, per the manual, are to have the positive (or negative) terminal tied directly to ground. I dont know if that changes anything. \$\endgroup\$ – user32763 Nov 15 '13 at 13:57
  • \$\begingroup\$ I don't know what a "rectifier shelf" is supposed to be. Obviously you can't tie both the positive and negative outputs of a power supply to ground. Without a diagram, this is too much hand waving. \$\endgroup\$ – Olin Lathrop Nov 15 '13 at 14:08

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